90 



USRL TEST STATIONS 



the assembly may be leveled after immersion. The 

 mounting of even the heaviest instruments on the 

 rotator does not require leveling because of its ri- 

 gidity. 



Determination of Test Conditions. Before an in- 

 strument is finally positioned, the depth, testing dis- 

 tance, and electric coupling are determined in as 

 close accord as possible with the principles set forth 

 in Chapter 5. It is evident that the final setup will 

 often be a compromise with ideal conditions. 



The depth of the testing areas is about 6 meters, 

 though most instruments are tested at 2 to 3 meters. 

 The exact depth may be selected with the hydro- 

 phone rod and the pipe suspensions. The length of 

 the hydrophone rod may be set approximately before 

 immersion, and adjusted afterward by the lead screw. 

 With the rotator, the adjustment of depth can only 

 be made in rather large steps and with no change pos- 

 sible after immersion. This requires that the associ- 

 ated testing instruments be adjusted to operate with 

 it, whatever its position. 



Estimates of testing distance based on instrument 

 size and frequency range are usually made before the 

 tests, so that the pier location of the instruments may 

 be tentatively determined. Several testing distances 

 are commonly used to observe the effects of the bot- 

 tom and surface reflections and of standing waves be- 

 tween the instruments. Projectors are usually faced 

 away from the shore, to avoid first-order shore reflec- 

 tions. 



When several projectors are mounted to calibrate 

 a sound field over a wide frequency range, the higher 

 frequency ones are mounted near the shore and di- 

 rected toward open water, while the lower frequency 

 devices are suspended at the far end of the pier and 

 directed toward shore. The test hydrophone is 

 mounted on the turret between the two projector 

 assemblies. This arrangement permits the hydro- 

 phone to be positioned with respect to either pro- 

 jector. An alternative method involves mounting the 

 two projectors back to back in the turret so that 

 either may be quickly set into proper relation to the 

 fixed hydrophone. Such an arrangement may be seen 

 in Figure 22. 



Usually the recommended electrical conditions are 

 approximated as closely as possible. If the device is a 

 projector, the recommended source impedance can 

 usually be matched with an available coil. If it is a 

 hydrophone, it may be connected directly to a high- 

 impedance coupling amplifier or terminated in ac- 



cordance with the recommended operating con- 

 ditions. 



Final Preparation for Testing. All too often the 

 leads furnished are too short to permit testing at the 

 proper depth. A number of single coaxial cables are 

 available for extending such leads. Splices are made 

 watertight by several layers of rubber tape or by the 

 use of underwater junction boxes. The cable is then 

 taped to the supporting rod and, in the case of heavy 

 cable, wound about the rod to prevent asymmetrical 

 loading. 



Before any instrument is tested, it is thoroughly 

 washed and debubbled, since significant errors may 

 be introduced by air bubbles or films. The active sur- 

 faces are washed with a soft cloth soaked in a strong 

 soap solution to which a generous supply of wetting 

 agent has been added. This procedure removes oil, 

 grease, or dirt particles which occlude air. The man- 

 ner in which the water meniscus traverses the instru- 

 ment face when it is lowered and raised in the water 

 is a good criterion of cleanliness. The meniscus pro- 

 gresses smoothly and without breaks if the face is 

 thoroughly wetted. A device having structural irregu- 

 larities which may trap air on, or near, the face is care- 

 fully debubbled after it is submerged by an air-free 

 stream of water from an underwater pump or hand 

 syringe. 



After an instrument has been rigged and thor- 

 oughly cleaned, it may still require soaking. This is 

 necessary to reach thermal equilibrium for x-cut Ro- 

 chelle salt crystals or others of high thermal inertia. 

 Such instruments are suspended in the water for sev- 

 eral hours before tests or even overnight, depending 

 on the size of the device and the difference between 

 air and water temperatures. 



The resistance of the device and the insulation 

 resistance between terminals and case are checked 

 before submersion and during tests if leakage is 

 suspected. 



After the instruments are positioned and electric 

 connections made, preliminary trials are made to de- 

 tect extraneous noise, cross talk, or signs of overload- 

 ing the hydrophone or receiving equipment. Exces- 

 sive noise is easily heard. Many test devices used in salt 

 water are not provided with electric shielding, and 

 so pick up power-line frequency from ground cur- 

 rents. When grounding adjustments do not improve 

 the signal-to-noise ratio sufficiently, it may be neces- 

 sary to insert suitable rejection filters ahead of the 

 wide-band receiving amplifier to prevent overload- 



